The Composition and Chemistry of Titan’s Atmosphere

Despite the relevance of the reactions of the prototypical nitrogen-containing six-membered aromatic molecule (N-heterocyclic) of pyridine (C6H5N) in environmental science, astrochemistry, planetary science, prebiotic chemistry, and materials science, few experimental/theoretical studies exist on the bimolecular reactions involving pyridine and neutral atomic/molecular radicals. We report a combined experimental and theoretical study on the elementary reaction of pyridine with excited nitrogen atoms, N(2D), aimed at providing information about the primary reaction products and their branching fractions (BFs). From previous crossed molecular beam (CMB) experiments with mass-spectrometric detection and present synergistic calculations of the reactive potential energy surface (PES) and product BFs we have unveiled the reaction mechanism. It is found that the reaction proceeds via N(2D) barrierless addition to pyridine that, via bridged intermediates followed by N atom “sliding” into the ring, leads to 7-membered-ring structures. They further evolve, mainly via ring-contraction mechanisms toward 5-membered-ring radical products and, to a smaller extent, via H-displacement mechanisms toward 7-membered-ring isomeric products and their isomers. Using the theoretical statistical estimates, an improved fit of the experimental data previously reported has been obtained, leading to the following results for the dominant product channels: C4H4N (pyrrolyl) + HCN (BF = 0.61 ± 0.20), C3H3N2 (1H-imidazolyl/1H-pyrazolyl) + C2H2 (BF = 0.11 ± 0.06), and C5H4N2 (7-membered-ring molecules or pyrrole carbonitriles) + H (BF = 0.28 ± 0.10). The ring-contraction product channels C4H4N (pyrrolyl) + HCN, C3H3N2 (1H-imidazolyl) + C2H2, C3H3N2 (1H-pyrazolyl) + C2H2, and C5H5 (cyclopentadienyl) + N2 have statistical BFs of 0.54, 0.09, 0.11, and 0.07, respectively. Among the H-displacement channels, the cyclic-CHCHCHCHNCN + H channel and cyclic-CHCHCHCHCN2 + H are theoretically predicted to have a comparable BF (0.07 and 0.06, respectively), while the other isomeric 7-membered-ring molecule + H channel has a BF of 0.03. Pyrrole-carbonitriles and 1H-ethynyl-1H-imidazole (+ H) isomeric channels have an overall BF of 0.03. Implications for the chemistry of Saturn’s moon Titan and prebiotic chemistry, as well as for understanding the N-doping of graphene or carbon nanotubes, are noted.

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Molecular Structure and Rotational Dynamics in the Acetonitrile:Acetylene (1:2) Plastic Co-crystal at Titan Conditions

Thakur,

Remsing

2024

ACS Earth Space Chem.

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The surface of Saturn's moon Titan is coated with small-molecule organic solids termed cryominerals. Cryominerals play an analogous role to minerals on Earth in Titan's surface geology and geochemistry. To develop a predictive understanding of Titan's surface geochemistry, we need to characterize the structure and dynamics of cryominerals at the molecular scale. We use ab initio molecular dynamics simulations to quantify the structure and dynamics of the acetonitrile:acetylene (1:2) cocrystal at Titan surface conditions. We suggest that acetonitrile:acetylene is in a plastic phase, in which acetonitrile molecules are dynamically disordered about the N−C−C axis on sub-picosecond timescales, and that this rotational, plastic disorder persists at least to temperatures of 30 K. We anticipate that many cryominerals may have plastic phases at or near Titan surface conditions, and understanding this disorder will be crucial to predicting chemistry on Titan's surface.

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The Composition and Chemistry of Titan’s Atmosphere

Cited by 5 publications

References 598 publications

The Challenge of Characterizing Bimolecular Reactions of Aromatic Compounds via Computational Chemistry. the Case of the $$\text {N(}^2$$D) + Pyridine Reaction and Implications in the Chemistry of the Thermosphere of Titan

The Challenge of Characterizing Bimolecular Reactions of Aromatic Compounds via Computational Chemistry. the Case of the $$\text {N(}^2$$D) + Pyridine Reaction and Implications in the Chemistry of the Thermosphere of Titan

Unveiling the Reaction Mechanism of the N(²D) + Pyridine Reaction: Ring-Contraction versus 7-Membered-Ring Formation Channels

Molecular Structure and Rotational Dynamics in the Acetonitrile:Acetylene (1:2) Plastic Co-crystal at Titan Conditions

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The Composition and Chemistry of Titan’s Atmosphere

Cited by 5 publications

References 598 publications

The Challenge of Characterizing Bimolecular Reactions of Aromatic Compounds via Computational Chemistry. the Case of the $$\text {N(}^2$$D) + Pyridine Reaction and Implications in the Chemistry of the Thermosphere of Titan

The Challenge of Characterizing Bimolecular Reactions of Aromatic Compounds via Computational Chemistry. the Case of the $$\text {N(}^2$$D) + Pyridine Reaction and Implications in the Chemistry of the Thermosphere of Titan

Unveiling the Reaction Mechanism of the N(2D) + Pyridine Reaction: Ring-Contraction versus 7-Membered-Ring Formation Channels

Molecular Structure and Rotational Dynamics in the Acetonitrile:Acetylene (1:2) Plastic Co-crystal at Titan Conditions

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Unveiling the Reaction Mechanism of the N(²D) + Pyridine Reaction: Ring-Contraction versus 7-Membered-Ring Formation Channels